The invention pertains to a system for monitoring the distance between a first and a second magnetically conductive part which are movable relative to one another, in particular, between a knife of a rotating cutterhead and a shearbar of a harvesting machine with at least one permanent magnet and one induction coil for generating an electric signal corresponding to the distance, which is supplied to an evaluation circuit, as well as a forage harvester with such a device.
It is desirable to monitor the distance between cutterhead knives and a shearbar in a forage harvesters, because this distance strongly affects power consumption. Several possibilities for measuring the distance between cutterhead knives and a shearbar are known. For example, in U.S. Pat. No. 3,646,421 capacitive sensors determine the capacitance between the knives and shearbars. In such a system a high voltage is necessary and consequently, the knives and shearbars must be electrically insulation from each other. Also, the capacitance may vary due to conditions other than a change in the distance between knife and shearbar. For example, a change in the nature of the material to be cut, and/or a change in dampness, can lead to capacitance fluctuations.
In EP-A-0,291,216 an acoustic knock sensor is proposed. It records the mechanical contacts of knife and shearbar. In the process, the "knock signals" are converted into digital signals. In order to permit a parallel positioning of the terminal edges of the knife and the shearbar, the two ends of the shearbar can be alternately moved up to the knife.
Other methods are based on a current measurement, as shown, for instance, in EP-A-0,679,330. When a constant supply voltage is applied, a current flows through the shearbar and a motor device which changes the position of the shearbar, a contact between a knife and the shearbar can be detected on the basis of a change in the current amplitude, since the ohmic resistance changes in this case. In case the current amplitude lies above or below prescribed limits within a given time interval, the operation of the motor is interrupted. In keeping with DE-OS-3,345,749, it is also possible to perform the distance measurement on the basis of a voltage breakdown between the shearbar and the knives. For this purpose, a high voltage is necessary.
In DE-A3,535,902, an optoelectronic sensor of distance measurement is proposed.
Additionally, the use of inductive sensors is known. These operate in two known manners. In the first design, incipient eddy currents are measured on the moved object. Inductive fields of very low strength are involved here. In the second, and more frequently employed design, also used in the present invention, induction coils are placed on the shearbar, in which a voltage is induced if existing magnetic fields are interrupted by the mutual motion of knives and the shearbar, as is shown, for instance, in EP-A-0,072,016 and U.S. Pat. No. 4,198,006. The large magnetic fields required for this are generated by one or more permanent magnets which are housed in the shearbar. The magnetic fields generated by the permanent magnet or magnets must be considerably larger than the magnetic fields generated by the intrinsic magnetization of the knives. The signals are dependent on the gap width and the velocity with which the knives pass the shearbar. It is possible with electronic integration devices to obtain a signal dependent on this velocity and to trigger the proximity sensors individually, insofar as several proximity sensors arranged along the shearbar. With an appropriate electronic signal evaluation, it is in principle possible to monitor whether knives are missing or broken off.
A problem with such a known system is that the induction coils are housed in the shearbar and consequently become worn or damaged if the shearbar is damaged. The permanent magnet or magnets are normally recessed in the shearbar and are embedded in plastic in order to avoid magnetic short circuits. This plastic housing becomes an additional wearing part. The embedding of the permanent magnet(s) and sensor coil(s) in the shearbar further reduces the mechanical stability of the shearbar, since a correspondingly large depression must be milconnected in the shearbar. Finally, a deterioration of the quality of the permanent magnets has an unfavorable effect on the sensitivity of the distance measurement.